1. Field of the Invention
The present invention relates to an optical imaging system for forming an image of a field of view in an object plane onto an image plane.
In conventional imaging optics, as illustrated by way of example in FIG. 1, a lens or mirror 1 is used to focus light rays 2 from an object plane 3, onto an image plane 4 and steps are generally taken to minimise optical distortions so as to produce a linear (i.e. proportional) relationship between areas of the object plane and areas of the image plane.
It is sometimes desirable to form an image on an image plane at an oblique angle to the object plane. One such arrangement is shown in FIG. 2 in which the image plane is designated by reference numeral 4 and the object plane by reference numeral 6. A practical application of this arrangement is a passive infrared intruder detector mounted on a wall, in which the object plane is the floor and the image plane is the surface of the detector. The detector is typically a two dimensional array of identical regularly spaced detector elements. It is clear from an examination of FIG. 2 that, assuming that the elements are identical and regularly spaced, certain elements of the detector array are viewing only a small area of ground whilst other elements are viewing very large areas of ground. In certain applications it would be advantageous to provide a more uniform correspondence between areas of the object plane and areas of the image plane.
Methods of correcting distortions in images when the object plane and image plane are not parallel are presented in, for example GB 2 298 497 A and U.S. Pat. No. 5,274,406. These are designed to generate a faithful reproduction of a rectangular photographic film, or other object, onto an inclined screen. In particular these methods correct for trapezoidal distortion and generate a rectangular image from a rectangular object.
There are occasions, however, when it is desirable to introduce a deliberate distortion into an image in order to enhance the performance of a detector. This may or may not be in conjunction with correcting distortions in a similar manner to the examples cited above. The problem is illustrated more clearly in FIG. 3. Using the configuration of FIG. 2, if a two dimensional array of 16×16 detector elements is used at the image plane, then the points 7 on the ground or object plane which are imaged onto the centres of the elements form an unevenly spaced array of points as illustrated in FIG. 3. (In FIG. 3 different symbols for points represent different rows of elements.) It is common practice to mount an intruder detector in a corner between two walls 8 and 9 and the requirement is to map a triangular area bounded by these walls as evenly as possible onto the square array.
This problem is not solved adequately using the prior art methods cited above as these would map a square area on the ground onto the array.